This invention relates generally to heterojunction bipolar transistors (HBTs) and, more particularly, to double HBT structures and techniques for achieving desirable characteristics in such structures. Briefly, by way of background, a heterostructure may be defined as a semiconductor structure in which the chemical composition varies with position in the structure. The simplest-heterostructure has a single heterojunction, i.e., a single interface within a semiconductor crystal across which the chemical composition changes. Examples include junctions between gallium antimony (GaSb) and indium arsenide (InAs) semiconductors, and junctions between indium phosphide (InP) and indium gallium arsenide (InGaAs). A common heterostructure with two heterojunctions is referred to as a double HBT, or DHBT. The electrical performance characteristics of the DHBT are highly dependent on the nature of the heterojunctions in the device.
In theory, an ideal heterojunction consists of a semiconductor crystal in which there exists a plane across which the identity of the atoms participating in the crystal changes abruptly. In practice, some crystal systems provide a close approximation of the atomically abrupt interface. In other crystals, there may be non-abrupt interfaces including crystalline defects near the interface. A semiconductor heterojunction interface may be made non-abrupt by design, by gradually varying the chemical composition of one of the materials near the interface. Such interfaces are referred to as graded heterojunctions.
An important concept in the theory and design of HBT structures is the energy band structure. In particular, it is important to define the concepts and inter-relationships of “conduction band,” “valence band” and “bandgap.” In semiconductors, the conduction band is the first empty energy band into which electrons must be promoted to contribute to conduction. The valence band is the energy band containing electrons that are shared in the process of valence bonding of atoms. In semiconductors, the valence band is full and the electrons in it are unable to contribute significantly to conduction, without being excited to the level of the conduction band. The bandgap is a band of “forbidden” energies between the top of the valence band and bottom of the conduction band. In general, electrons do not have stable states with energies within the bandgap. A useful tool for predicting and analyzing HBT performance is the energy band diagram, which plots the extent of the bandgap energy versus position in the device. The upper edge of the bandgap (the bottom of the conduction band) is usually referred to as having an energy EC, measured with respect to a common energy reference, and lower edge of the bandgap (the top of the valence band) is usually referred to as having an energy EV, with respect to the same energy reference.
In the materials on each side of a heterojunction, the bandgaps are usually different. Therefore, the energy of charge carriers at at least one of the edges of the bandgap must change as those carriers pass through the heterojunction. These discontinuities may exist in the conduction band, the valence band, or both, depending on the manner in which these bands align or misalign at the heterojunction. Heterojunction band alignment is sometimes categorized in the technical literature as being of Type I, Type II or Type III. A Type I heterojunction is one in which the bandgap of one of the materials is completely contained within the bandgap of the other. This is sometimes referred to as a “straddling” relationship. In a Type II heterojunction, one bandgap partially overlaps the other, which is sometimes referred to as a “staggered” relationship. In a Type III heterojunction, the adjacent bandgaps do not overlap at all, which is sometimes referred to as a “broken-gap” relationship.
FIGS. 1–3 are examples of bandgap diagrams for prior art DHBT structures. FIG. 1 is a pair of bandgap diagrams for a Type I abrupt junction npn DHBT structure in which the emitter and collector materials are both indium phosphide (InP) and the base material is indium gallium arsenide (InGaAs). The left-hand portion of the bandgap diagram shows the bandgaps of the materials when they are isolated, and the right-hand portion of the diagram shows the bandgaps of the materials when they have been “joined” at the two heterojunctions. It will be understood, of course, that there is no “joining” step as such, because the entire structure is formed as a single crystal by an epitaxial process. In each case, the horizontal axis plots position within the device, measured in a direction normal to the heterojunction interfaces.
When the materials on each side of a heterojunction are “joined,” charge carriers will flow across the junction, building space charges until the Fermi energy is the same everywhere in the material. At positions far removed from the junction, the bandgap diagram remains unchanged. To represent this condition in a bandgap diagram, one first aligns the two Fermi energy levels (not shown in the figures, but always falling somewhere within the bandgap). Then, one must adjust the edges of the bandgap to accommodate the new alignment, but maintaining the bandgap of each material constant. As illustrated in FIG. 1, in some cases it is impossible to do this without introducing a discontinuity or cusp in either the conduction band edge or the valence band edge. The cusp at the base-emitter junction results in an increased base-emitter turn-on voltage because electrons need to have a certain energy before being transmitted across the junction. The cusp at the base-collector junction results in an electron blocking effect, preventing electrons from being transported from the base region into the collector at small base-collector voltage. Both of these phenomena have undesirable effects on device performance, because they require a large bias voltage at both the emitter-base and base-collector junctions.
Using graded junctions in a Type I device is depicted in FIG. 2. In this structure, the emitter material is indium aluminum arsenide (InAlAs), the base material is indium gallium arsenide (InGaAs) and the collector material is indium phosphide (InP). The gradual material transitions in the emitter-base junction lead to a lower turn-on voltage, and the graded base-collector junction reduces the electron blocking effect. However, the resulting bandgap diagram has a significant dip in the conduction band of the collector, blocking some electrons that could otherwise be passing to the collector output. Therefore, the major drawback of this device structure is that electron blocking from the base to the collector still occurs at small base-emitter bias voltages. In effect, the use of graded junctions reduces the magnitudes of the cusps or discontinuities, but they are still present to some degree.
A Type II heterojunction HBT structure is depicted in FIG. 3. The emitter material is indium phosphide (InP), the base material is gallium arsenide antimonide (GaAsSb) and the collector material is indium phosphide (InP). This choice of base material raises the base conduction band above that of the emitter and collector conduction band levels. This allows electrons to pass more freely from the base to the collector. However, the drawbacks are the difficulty of fabricating abrupt InP/GaAsSb heterojunctions and the increase in the emitter-base turn-on voltage, so that a large emitter-base bias voltage must be used.
It will be appreciated from the foregoing that all of these described DHBT structures of the prior art have significant disadvantages. Accordingly, a DHBT structure without these disadvantages is still needed. The present invention satisfies this need.